The omega-3 fatty acids are necessary for maintaining cellular functional integrity, and are necessary in general for human health. Docosahexaenoic acid (22:6 n-3, DHA), an important omega-3 component of fish oil and of marine algae, is concentrated in the brain, in the photoreceptors and in the synapses of the retina. DHA-enriched diets are initially metabolised by the liver and afterwards distributed via the lipoproteins in the blood in order to meet the needs of the various organs. The administration of DHA leads to an increase of its concentration at tissue level, inducing also an increase in the concentration of omega-3 eicosapentaenoic acid (EPA) which in linked metabolically, whereas the administration of EPA only increases its concentration decreasing that of DHA at cell level.
In general, the DHA is incorporated into the phospholipids of the cell membrane, which have effects on its composition and functionality, on the production of reactive oxygen species (ROS), on membrane lipid oxidation, on transcription regulation, on the biosynthesis of eicosanoids and on intracellular signal transduction. Furthermore, in the central nervous system, the DHA is involved in the development of the learning capacity related to the memory, in the excitable functions of the membrane, in biogenesis of the photoreceptor cells and in transducing the signal dependent upon quinase protein. A potential dietary therapy would be based on correcting the optimum levels of omega-3 fatty acids to prevent certain pathologies from originating or progressing, such as inflammatory pathologies, tumoral processes, cardiovascular diseases, depression and neurological disorders.
In the central nervous system, both the brain and the retina show an unusual capacity for retaining DHA, even under situations of very prolonged dietary deficiencies of omega-3 fatty acids. Several studies have described the protective effect of DHA on neurones, in which it is present in very high levels. For example, it is involved in protecting the neuronal cells from death by apoptosis. Recently, it has been shown that DHA, found in reduced amounts in the hippocampus of rats of advanced age, is capable of protecting primary cultures of said cells against the cytotoxicity induced by glutamate.
In the photoreceptors of the retina, DHA has also been shown to modulate the levels of the pro- and anti-apoptotic proteins of the Bcl-2 family. The external segments of the retinal photoreceptor contain rodopsin, as well as a higher DHA content than any other type of cell. The DHA is concentrated in the phospholipids of the photoreceptor segment disc's outer membranes. Retinal dysfunctions have been observed under conditions of reduction of optimal DHA concentration. The retina pigmentary epithelial cell (RPE) plays a very active role in DHA take-up, conservation and transport. The high DHA content in the photoreceptor and in the RPE cells is mainly linked to domains in the membrane with physical characteristics that contribute to the modulation of receptors, ionic channels, carriers, etc., while it also appears to regulate the concentration of phosphatidilserine.
It is unknown to date if these effects are entirely mediated by the DHA itself or by any metabolic derivatives. Certain derivatives of DHA have been identified in the retina. Although the enzymes involved in the synthesis of said derivatives have not been identified precisely, some recent results suggest the participation of an A2 phospholipase (PLA2) followed by a lipoxygenase (LOX). The PLA2 releases the DHA from the membrane phospholipids and the LOX converts it into its metabolically active derivatives.
The reactive oxygen species (ROS) are produced during normal cellular functioning. The ROS include the superoxide anion, hydrogen peroxide and the oxydril radical. Their high chemical reactivity leads to the oxidation of proteins, of DNA or of lipids. The superoxide dismutase (SOD), the catalase (CAT) and the glutation peroxidase (GPx) are the primary antioxidant enzymes that protect against the molecular and cellular damage caused by the presence of ROS. The oxidative stress activates many metabolic channels; some are cytoprotective, while others lead to death of the cell. Recent studies indicate that an imbalance between ROS production and breakdown is a significant risk factor in the pathogenesis of many illnesses, in some cases related to a deterioration of the antioxidant system.
The DHA is presented as a target of the ROS that produces damage to the cell of the photoreceptor and to the RPE. The retinal degeneration induced by light promotes loss of DHA in the photoreceptors. For example, when the RPE cells are damaged or die, photoreceptor function deteriorates because the RPE cells are essential for its survival. Thus, death of the RPE cell under the effect of oxidative stress leads to a deterioration of eyesight, particularly when the cells of the macula are affected, since it is responsible for eyesight acuity. The pathophysiology of many retinal degenerations (e.g., macular degenerations related to age and to Stargardt disease) involves oxidative stress that leads to RPE cell apoptosis. Indeed, RPE cell apoptosis appears to be the dominant factor in the macular degeneration observed with age. Such studies suggest that said cells have developed highly effective antioxidant mechanisms to protect themselves from their high DHA content and show notable adaptive capacity.
Furthermore, the relationship between the free radicals and ageing is perfectly well accepted, based on the evidence that free radicals produced during aerobic respiration cause oxidative damage that accumulates and leads to a gradual loss of the homeostatic mechanisms, interference in gene expression patterns and a loss of the cell's functional capacity, leading to ageing and death. An interrelation exists between the generation of oxidants, antioxidant protection and repair of the oxidative damage. Many studies have been carried out to determine whether antioxidant defences decline with age. These have included analysis of the main components thereof: activity or expression of the SOD, CAT, GPx enzymes, glutation reductase, glutation-S-transferase and the concentration of compounds of low molecular weight with antioxidant properties. For example, an over-expression of SOD and CAT in Drosophila melanogaster increases life expectancy by 30% and reduces damage by protein oxidation. In this context, in vitro and in vivo exposure of cutaneous tissue to UV rays generates free radicals and other reactive oxygen species, leading to cellular oxidative stress, documented as contributing significantly to ageing. Excessive exposure of the skin to ultraviolet radiation can give rise to acute or chronic damage. Under acute conditions erythema or burns can be produced, while chronic over-exposure increases the risk of skin cancer and ageing. Moreover, it is known that the cutaneous cells can respond to acute or chronic oxidative stress by increasing expression of a variety of proteins, such as the enzymes involved in maintaining cell integrity and resistance to oxidative damage.
In the art, it is well known that telomeres are non-coding DNA regions located at the ends of eukaryotic chromosomes. These are constituted by highly conserved DNA sequences, repeated in tandem (TTAGG)n and associated proteins, and have a special structure which hinders the ligation to the ends of other chromosomes, preventing the telomeric fusion. They have an essential role in the preservation of the chromosomic integrity, protecting the coding DNA from the enzymatic action and its degradation, contributing to the maintenance of the chromosomic stability.
In contrast with coding sequences which have a semiconservative replication, the telomeres undergo a progressive loss of its repetitive sequences during the successive cell division. Nowadays, it is considered that a minimum telomeric length is required in order to keep the telomere function and when these reach a critical size they have difficulties for the division in the mitosis, generating telomeric association (TAS) and chromosomic instability. Said chromosomic instability would be associated with an increase in the probability of producing errors capable of generating significant genetic changes.
Owing to the multiplicity of double bonds, the omega-3 fatty acids are considered to be molecular targets for generation and propagation of free radicals during the oxidative stress processes related to generation of lipidic peroxides. Contradictory results have been obtained, however, in various studies of susceptibility to oxidative stress owing to dietary supplements of omega-3 fatty acids. Some studies in humans have shown increased oxidation of the LDL, while others have found no such effect. In studies with animals, treatment with omega-3 fatty acids has been found to lead to increased or reduced susceptibility to oxidation of the LDL. On the other hand, an over-expression of the genes involved in the antioxidant defence system has been found in the livers of mice fed on a fish-oil-enriched diet for three months.
Furthermore, various in vitro studies with a cellular line of glyal origin have shown that membranes rich in omega-3 fatty acids are more susceptible to oxidative damage. Long-term supplementation of these cells with high concentrations of DHA resulted in increased levels of lipidic peroxides in the culture medium, and a higher percentage of cell death due to apoptosis induced by exposure to hydrogen peroxide. It has also been shown, however, that intra-amniotic administration of ethyl docosahexaenoate reduces lipidic peroxidation in the foetal brains of rats. It has been suggested that this response is due to a free-radical sequestering effect via activation of antioxidant enzymes. An increase in the antioxidant capacity of the brain is important for the primary endogenous defence against oxidative stress, because the brain is relatively rich in polyunsaturated fatty acids and relatively poor in antioxidant enzymes.
These contradictory results suggest that the hypothesis based on the premise that oxidation of a fatty acid increases with the number of double bonds has no in vivo applicability, since other potential mechanisms may act to reduce oxidative damage, such as a three-dimensional structure of the omega-3 fatty acids in the lipids and lipoproteins of the membrane that make the double bonds less susceptible to an attack by the ROS, an inhibition of pro-oxidant enzymes such as PLA2 or a greater expression of antioxidant enzymes.
On the other hand, the idea of associating physical exercise with the production of free radicals comes from early 80s due to the observation of the damage in membrane lipids during ischemia-reperfusion events in hypoxic tissue (see Lovlin et al., Eur. J. Appl. Physiol. Occup. Physiol. 1987, 56 (3) 313-6). At the same time, an increase in the GSSH/GSH ratio was observed in rat muscle cells (see Lew H. Et al. FEBS Lett, 1985; 185(2): 262-6, Sen C K et al., J. Appl. Physiol. 1994; 77(5): 2177-87) as well as in human blood (see MacPhail Db et al., Free Radic Res Commun 1993; 18(3): 177-81, Gohil K. et al. J. Appl. Physiol. 1988 January; 64(1): 115-9). Free radicals also affect DNA and acute physical exercise increases damage in DNA, as evidenced by the increase of 8-OxodG. Exhausting physical effort (running a marathon) causes damage in DNA which is evident for some days after the trial and also causes damage in immunocompetent cells (which can be associated with the immune decrease shown in sportsmen after such a trial).
However, other authors did not observe any effects (except for minor damage) after swimming for 90 minutes, running for 60 minutes or making an exhausting effort by rowing. At the same time, researches on trained and non-trained sportsmen did not find any difference in the urinary excretion of 8-oxo-dG, even those finding such a damage, considered to be secondary to subsequent reactions to the effort and not to the action of exercise over the DNA in acute way.
The event of intensive physical exercise producing oxidative stress is very well known in the art, but its origin is not well determined yet.
Studies carried out with n-3 fatty acids related to sports performance were focused on the antiinflammatory effect and, indeed, first assays tried to find the possible action of these nutrients improving the alveolar-capillary absorption by diminishing the intensive physical exercise-induced broncoconstriction. In that regard, Mickleborough proved that after administering 3.2 g EPA and 2.2 g DHA regime proinflammatory cytokines were attenuated by diminishing the presence of TNF-α and IL-1β in an elite athlete, along with a decrease in the broncoconstriction. Walser related n-3 fatty acids vascular effects to positive effects in people showing intolerance to physical exercise. In that regard, van Houten et al. studied that a n-3 fatty acid high ingestion was associated with a better recovery in patients carrying out a cardiac rehabilitation after a coronary syndrome.
The absence of positive results in the physical performance in the analyzed studies is due to the evaluation of patients, not healthy people, and what it has been searched are vascular and inflammatory effects.
At the same time, researches have been carried out based on the following theoretical concept: increasing free fatty acids in plasma above 1 mmol/L (occurring when glycogen is used up), the competence with tryptophan transport makes this to be increased with the subsequent serotonine increase, a neurotransmitter related to the so-called “central fatigue” in long duration sports. In that regard, it is known that n-3 fatty acids diminish the amount of free fatty acids in plasma probably by up-regulating the fatty acid oxidation by activating the transcription nuclear factor PPARα. However, these assays were not successful, since Huffman (2004) by using a dose regime of 4 g of n-3 fatty acid (500 g capsules containing 300 mg EPA and 200 mg DHA) carried out a study in both sex runners without finding any decrease in free TRP nor a less perception of effort, nor any statistically performance increase in the performance, although there was a statistical tendency for improving the performance in subjects whom n-3 fatty acid were administered, leaving open the possibility to authors that the cause of diminishing the statistical power for the study was the low number of subjects studied (5 men and 5 women).
Another subsequent research wherein the efficacy of n-3 acids related to the performance was evaluated did not find any significant differences using maize oil as a placebo. Raastad administering 1.60 g EPA and 1.04 g DHA per day for several weeks, did not find any improvement in football players (see, Raastad et al. Scand J. Med Sci Sports 1997; 7(1): 25-31).
On the other hand, it is known that free fatty acids interfere with the use of glucose in the muscle, since its analogues at intracellular level, acyl-CoA, in the mitochondria inhibit the pyruvate dehydrogenase (inhibition by product), furthermore, stimulates glycogenolysis and glyconeogenesis, causing a smooth hyperglycemia during fasting, indeed, the continuous administration of polyunsaturated fatty acids during fasting helps to maintain glycemia, by maybe activating glucose-6-phosphatase at a hepatic level. It is also known that a composition of fatty acids in the muscle alters insulin sensitivity, showing that a high content of polyunsaturated fatty acids in plasmatic membrane improves insulin sensitivity and a high content of saturated fatty acids produces the opposite effect.
Exercise increases glucose uptake, capillary perfusion, glycogen synthesis rate and insulin sensitivity. During muscular contraction changes are produced in temperature, intracellular pH, ATP/ADP ratio, as well as Ca++ intracellular concentration and other metabolites which could act as messengers in the cellular functioning regulation with exercise. In this regard, Ca++ regulates a great amount of intracellular proteins, including calmodulin kinase, protein kinase C(PKC) and calcineurin which are important intermediates in the signals of intracellular transduction. During aerobic exercise, acetyl-CoA carboxylase is deactivated by AMP kinase (AMPK) which leads to a drop in malonyl-Coa levels, deinhibiting carnitine palmitol transferase with the resulting increase of fatty acid transport within the mitochondria (thus promoting fatty acid oxidation).
AMPK activation effects probably include stimulation of GLUT4 and hexokinase expression, as well as mitochondria enzymes. However, surprisingly, AMPK activation is not the unique way (independent of insulin) wherein the exercise increases the response to glucose in skeletal muscle. See Mora and Pessin, J. Biol. Chem. 2000; 275 (21): 16323-16328, showed that an increase in the glucose response in the muscle, indeed, there are several transcription factors such as MEF2A and MEF2D activating GLUT4 and those factors are activated by exercise.
An increase in intramuscular lipids is common in obesity states and physical training, but the result is that for obese people is associated with insulin resistance, whereas in sportsmen the great activity of carnitine palmitol transferase makes fatty acids undergo beta oxidation. There are strong evidences that a rich diet in n-3 fatty acid, even with an increase of glycemia and insulinaemia (signals of insulin resistance), act at a insulin receptor level maintaining the level of GLUT-4 protein translocation, which has specifically showed with DHA (see, Jaescchke H. Proc. Soc Exp Biol. Med 1995; 209: 104-11).